The vertical boat method (vertical gradient freeze (VGF method)) and the vertical Bridgeman method (VB method) are used to carry out crystal growth in order to produce n-type conductivity GaAs single crystal ingots for producing GaAs wafers, which are materials for GaAs (gallium arsenide) devices.
In this case, Si is added as a dopant to the GaAs single crystal in order to provide a carrier to the ingot when an n-type electroconductive ingot is produced. On the other hand, there is a method in which B2O3 (boric oxide) is used as a liquid sealant for preventing As, which is a volatile component, from dissociating from the ingot, and for achieving other goals when the ingot is produced by the vertical boat method. However, since there is a reaction between B2O3 and the Si added as a dopant, a phenomenon occurs in which the Si concentration in the ingot becomes difficult to control. As a result, it is difficult to consistently and stably produce an ingot having a desired favorable carrier concentration distribution. In order to solve this problem, the present inventors have proposed in Patent Document 1 the addition of a suitable amount of Si to the liquid sealant in advance, and have proposed in Patent Document 2 the production of an ingot while the liquid sealant is stirred.
On the other hand, since there is a demand for ingots thus produced to have good crystallinity, the present inventors have proposed in Patent Document 3 a method in which a crucible is used to hold GaAs raw material synthesized into a shape that substantially conforms to the internal shape of the crucible, B2O3 is furthermore placed in the crucible, and the crucible is then heated to fuse the GaAs raw material, the dopant Si raw material, and the B2O3.
Proposed in Patent Document 4 is a method in which a seed crystal, raw material, and liquid sealant are placed in a bottomed cylindrical growth container, the raw material is heated and melted using a prescribed temperature distribution with the aid of an electric furnace heater disposed so as to surround the growth container, and a compound semiconductor single crystal is vertically grown in order to dispense with the need to provide a furnace temperature distribution designed to make the dopant concentration uniform. In the method, a hole is opened in the block-shaped raw material, a dopant is loaded into the hole, and the raw material is introduced into the growth container to grow an n-type or p-type compound semiconductor single crystal.    Patent Document 1: JP-B 3-57079    Patent Document 2: JP-A 2000-109400    Patent Document 3: JP-A 2004-137096    Patent Document 4: JP-A 2004-345888